Parsing of Ordered Timestamps in Local Time (to UTC) While Observing Daylight Saving Time

Question

In C#:

// Define the input values.
string[] input =
{
    "2013-11-03 00:45:00",
    "2013-11-03 01:00:00",
    "2013-11-03 01:15:00",
    "2013-11-03 01:30:00",
    "2013-11-03 01:45:00",
    "2013-11-03 01:00:00",
    "2013-11-03 01:15:00",
    "2013-11-03 01:30:00",
    "2013-11-03 01:45:00",
    "2013-11-03 02:00:00",
};

// Get the time zone the input is meant to be interpreted in.
TimeZoneInfo tz = TimeZoneInfo.FindSystemTimeZoneById("Eastern Standard Time");

// Create an array for the output values
DateTimeOffset[] output = new DateTimeOffset[input.Length];

// Start with the assumption that DST is active, as ambiguities occur when moving
// out of daylight time into standard time.
bool dst = true;

// Iterate through the input.
for (int i = 0; i < input.Length; i++)
{
    // Parse the input string as a DateTime with Unspecified kind
    DateTime dt = DateTime.ParseExact(input[i], "yyyy-MM-dd HH:mm:ss",
                                      CultureInfo.InvariantCulture);

    // Determine the offset.
    TimeSpan offset;
    if (tz.IsAmbiguousTime(dt))
    {
        // Get the possible offsets, and use the DST flag and the previous entry
        // to determine if we are past the transition point.  This only works
        // because we have outside knowledge that the items are in sequence.
        TimeSpan[] offsets = tz.GetAmbiguousTimeOffsets(dt);
        offset = dst && (i == 0 || dt >= output[i - 1].DateTime)
                 ? offsets[1] : offsets[0];
    }
    else
    {
        // The value is unambiguous, so just get the single offset it can be.
        offset = tz.GetUtcOffset(dt);
    }

    // Use the determined values to construct a DateTimeOffset
    DateTimeOffset dto = new DateTimeOffset(dt, offset);

    // We can unambiguously check a DateTimeOffset for daylight saving time,
    // which sets up the DST flag for the next iteration.
    dst = tz.IsDaylightSavingTime(dto);

    // Save the DateTimeOffset to the output array.
    output[i] = dto;
}


// Show the output for debugging
foreach (var dto in output)
{
    Console.WriteLine("{0:yyyy-MM-dd HH:mm:ss zzzz} => {1:yyyy-MM-dd HH:mm:ss} UTC",
                       dto, dto.UtcDateTime);
}

Output:

2013-11-03 00:45:00 -04:00 => 2013-11-03 04:45:00 UTC
2013-11-03 01:00:00 -04:00 => 2013-11-03 05:00:00 UTC
2013-11-03 01:15:00 -04:00 => 2013-11-03 05:15:00 UTC
2013-11-03 01:30:00 -04:00 => 2013-11-03 05:30:00 UTC
2013-11-03 01:45:00 -04:00 => 2013-11-03 05:45:00 UTC
2013-11-03 01:00:00 -05:00 => 2013-11-03 06:00:00 UTC
2013-11-03 01:15:00 -05:00 => 2013-11-03 06:15:00 UTC
2013-11-03 01:30:00 -05:00 => 2013-11-03 06:30:00 UTC
2013-11-03 01:45:00 -05:00 => 2013-11-03 06:45:00 UTC
2013-11-03 02:00:00 -05:00 => 2013-11-03 07:00:00 UTC

Note that this assumes that the first time you encounter an ambiguous time like 1:00 that it will be in DST. Say your list was truncated to just the last 5 entries – you wouldn’t know that those were in standard time. There’s not much you could do in that particular case.

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